1. Field of the Invention
The present invention relates to thick film electrodes and multilayer ceramic electronic devices. In particular, the present invention relates to a thick film electrode disposed on a surface of any one of various electronic devices, such as multilayer ceramic electronic devices, and a multilayer ceramic electronic device, such as a multilayer piezoelectric device including the thick film electrode as an external electrode.
2. Description of the Related Art
In a conventional multilayer ceramic electronic device, such as a multilayer piezoelectric device, thick-film external electrodes primarily composed of a conductive material, such as Ag, are disposed at both end surfaces of a multilayer piezoelectric component in which an internal electrode is embedded.
In general, the external electrodes are formed by printing or applying a conductive paste on the multilayer piezoelectric component and then performing firing. The conductive paste is formed by kneading a conductive powder, such as Ag, a glass frit, an organic resin, and an organic solvent.
However, in such a multilayer ceramic electronic device, repeated expansion and contraction of the ceramic laminate during polarization treatment or in the subsequent operation for a prolonged period causes the ceramic laminate to fatigue, thus resulting in a crack. Then, the crack extends to the external electrode, thereby fracturing the external electrode. As a result, the multilayer ceramic electronic device does not function as an electronic device.
Accordingly, a known multilayer ceramic capacitor-type electrostrictive device has been proposed, the device being produced by bringing a conductive component, such as a metal plate, steel wool, or a conductive rubber, into contact with the side surface of an external electrode, covering the conductive component with a heat-shrinkable tube, and shrinking the heat-shrinkable tube with a hot blast (see, for example, Japanese Unexamined Patent Application Publication No. 5-218519).
Japanese Unexamined Patent Application Publication No. 5-218519 attempts to ensure the electrical connection by bonding the conductive component to the external electrode, even if a crack occurs in the laminate or the external electrode.
In another known technique, a multilayer piezoelectric actuator device has been proposed, the device including a pair of conductive members and a pair of external electrodes. Each conductive member is spaced from and facing the corresponding side surface, and each conductive member is connected to the corresponding external electrode (see, for example, Japanese Unexamined Patent Application Publication No. 2002-9356).
In Japanese Unexamined Patent Application Publication No. 2002-9356, one end of the conductive member in the width direction is bonded to the external electrode by brazing, and the other end of the conductive member in the width direction is a free end. Even if a crack occurs in the laminate or the external electrode, this structure avoids the extension of the crack to ensure the continuity of the conductive member, thereby preventing a deterioration in function.
However, Japanese Unexamined Patent Application Publication No. 5-218519 has the following problems: as shown in FIG. 7, the conductive member 53 is merely bonded to the external electrode 52 with the heat-shrinkable tube 51. That is, the conductive member 53 is not integrally formed with the external electrode 51, which results in discontinuous adhesion at a bonded surface 54. Thus, when a crack 56 occurs in a ceramic laminate 55 because of, for example, a continuous operation for a prolonged period, a tensile stress acts in the directions of Arrows a and a′. As a result, the external electrode 52 may be ruptured, thus leading to poor electrical continuity.
Japanese Unexamined Patent Application Publication No. 5-218519 has further problems as described below: the external electrode 52 is bonded to the conductive member 53 as described above. That is, the contact between the external electrode 52 and the conductive member 53 establishes the continuity. Thus, if the device is subjected to high temperature and high humidity for a prolonged period of time, the conductivity disadvantageously decreases.
Furthermore, Japanese Unexamined Patent Application Publication No. 2002-9356 has the following problems: since the external electrode is not integrally formed with the conductive member, the device has low mechanical strength. Thus, similarly to Japanese Unexamined Patent Application Publication No. 5-218519, the occurrence of a crack in the laminate or the external electrode results in continuity failure. Moreover, if the device is subjected to high temperature and high humidity for a prolonged period of time, the conductivity disadvantageously decreases.
Japanese Unexamined Patent Application Publication No. 2002-9356 has further problems as follows: since the external electrode is bonded to the conductive member by brazing, stress is concentrated on the interface between the external electrode and the conductive member. Thus, detachment is likely to occur at the interface.